IntersectionObserver: Always send an initial notification.

third_party/WebKit/Source/core/dom/IntersectionObservation.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/IntersectionObservation.h"
 
 #include "core/dom/ElementRareData.h"
 #include "core/dom/IntersectionObserver.h"
 #include "core/layout/IntersectionGeometry.h"
 
 namespace blink {
 
 IntersectionObservation::IntersectionObservation(IntersectionObserver& observer,
                                                  Element& target,
                                                  bool shouldReportRootBounds)
     : m_observer(observer),
       m_target(&target),
       m_shouldReportRootBounds(shouldReportRootBounds),
-      m_lastThresholdIndex(0) {}
+      // Note that the spec says the initial value of m_lastThresholdIndex
+      // should be -1, but since m_lastThresholdIndex is unsigned, we use a
+      // different sentinel value.
+      m_lastThresholdIndex(kMaxThresholdIndex - 1) {}
 
 void IntersectionObservation::computeIntersectionObservations(
     DOMHighResTimeStamp timestamp) {
   DCHECK(observer());
   if (!m_target)
     return;
   Vector<Length> rootMargin(4);
   rootMargin[0] = m_observer->topMargin();
   rootMargin[1] = m_observer->rightMargin();
   rootMargin[2] = m_observer->bottomMargin();
@@ skipping 26 matching lines @@
           geometry.intersectionRect().size().height().toFloat();
       float targetArea = geometry.targetRect().size().width().toFloat() *
                          geometry.targetRect().size().height().toFloat();
       newVisibleRatio = intersectionArea / targetArea;
     }
     newThresholdIndex = observer()->firstThresholdGreaterThan(newVisibleRatio);
   } else {
     newVisibleRatio = 0;
     newThresholdIndex = 0;
   }
+
+  RELEASE_ASSERT(newThresholdIndex < kMaxThresholdIndex);
+
   if (m_lastThresholdIndex != newThresholdIndex) {
     IntRect snappedRootBounds = geometry.rootIntRect();
     IntRect* rootBoundsPointer =
         m_shouldReportRootBounds ? &snappedRootBounds : nullptr;
     IntersectionObserverEntry* newEntry = new IntersectionObserverEntry(
         timestamp, newVisibleRatio, geometry.targetIntRect(), rootBoundsPointer,
         geometry.intersectionIntRect(), target());
     observer()->enqueueIntersectionObserverEntry(*newEntry);
     setLastThresholdIndex(newThresholdIndex);
   }
 }
 
 void IntersectionObservation::disconnect() {
   DCHECK(observer());
   if (m_target)
     target()->ensureIntersectionObserverData().removeObservation(*observer());
   m_observer.clear();
 }
 
 DEFINE_TRACE(IntersectionObservation) {
   visitor->trace(m_observer);
   visitor->trace(m_target);
 }
 
 }  // namespace blink